A photoelectric switch as shown in FIG. 6 is known. In FIG. 6, a light emitting means comprising an oscillation circuit 1 and a light emitting diode 2, connected to the output terminal of the oscillation circuit 1. A light detecting means, on the other hand, comprises an amplifier circuit 4, provided with a photo transistor 3, that receives a light pulse signal emitted from the light emitting diode 2 and converts the received light pulse signal into an electrical signal. Amplifier circuit 4 amplifies the signal and a capacitor 5 is connected between the output terminal of amplifier circuit 4 and the input terminal of a comparator 6. Detection circuit 7 is connected to the output terminal of comparator 6.
FIGS. 7(a)-(d) show waveforms at various portions of the photoelectric switch. The oscillation circuit 1 generates a pulse current as shown in FIG. 7(a) to actuate light emitting diode 2 to emit a light pulse signal. The amplifier circuit 4 receives the light pulse signal from light emitting diode 2 and converts it into an electrical signal by means of photo transistor 3, and then amplifies the electric signal to make it have a waveform as shown in FIG. 7(b). Capacitor 5 cuts a DC component of the amplified signal, and transfers the signal to comparator 6. The comparator 6 discriminates the signal applied thereto on the basis of a predetermined level to obtain a digital signal suitable for logical processing as shown in FIG. 7(c). Detection circuit 7 generates a detection signal as shown in FIG. 7(d) when the pulse output of comparator 6 is applied continuously a predetermined number of times.
The conventional photoelectric switch, however, has a problem in that, when a plurality of light emitting means 20A and 21A are arranged adjacent to each other, and a plurality of light detecting means 20B and 21B are arranged adjacent to each other, as shown in FIG. 8, light detecting means 20B receives a light signal from the light emitting means 21A, although a light signal from the light emitting means 20A is blocked off by an object 22 so that the light detecting means 20B cannot detect existence of the object 22. In an arrangement of a plurality of photoelectric switches, therefore, it is necessary to sufficiently separate the optical axes of the photoelectric switches from each other.